The present disclosure is directed to defect detection in objects, and more particularly to defect detection using statistical analyses and procedures performed on output responsive to detected portions of waves (e.g., acoustic waves) traveling in the objects.
Conventional rail inspection techniques rely on the use of electromagnetic (EM) induction testing, or ultrasonic testing (UT) inspection, and the processing of acquired data generally using non-statistical techniques. Examples of non-statistical techniques used to inspect rails are those of ultrasonic pulse-echo or through-transmission testing, in which a defect is detected by the appearance of an ultrasonic echo (pulse-echo) or a drop in ultrasonic amplitude (through-transmission). Such analysis relies on one individual signal, without any statistical analysis. These analyses are therefore deterministic, i.e. they are affected by the inherent signal variability due to realistic environmental and operational conditions which greatly limits the reliability of defect detection achievable. The use of EM and UT techniques together with the processing of acquired data using non-statistical analysis procedures often leads to occurrences of both “false positives” (e.g., undamaged regions of the object, such as a rail, being erroneously flagged as cracked), and “false negatives” (e.g., defects that are not detected). False negatives are of particular concern because failure to detect defects in rail tracks may lead to train derailments. For example, the above inspection techniques have traditionally had difficulties in detecting internal cracks that are located under a subsurface discontinuity known as shelling (FIG. 1A). The latter type of defect was determined to be responsible, for example, for two major derailments in Superior, Wis., in 1991, and Hatfield, UK, in 2000.
Transverse cracks in the railhead are considered to be amongst the most serious defects found in rails, and are deemed to have been responsible for 541 derailments and $91 Million in direct cost during the period 1992-2002 in the US according to Federal Railroad Administration Safety Statistics. The associated indirect cost of these accidents is even higher. Other defects in rail are challenging to detect by conventional non-statistic inspections. These include vertical split head defects, horizontal split head defects, vertical split web defects, weld defects, and bolt hole cracks, as shown in FIG. 1B.
Another drawback of conventional inspection techniques, when applied to objects such as rails, is the relatively slow speed at which the inspection techniques can be performed. Typical inspection speed of rails, for example, using conventional systems/techniques is less than 20 miles per hour (mph).